Move exitQueue cache values to epochContext

[dapplion]

Motivation

To speed up validator exits @tuyennhv added an optimization in #2695 to loop over validators one per exits on a block. This optimization can be taken further since the expensive computations can be done once per epoch only. Also the exitQueueEpoch and churn are already computed in epoch process.

Description

Moves exitQueueEpoch, exitQueueChurn and churnLimit cached values from blockProcess and epochProcess to epochContext. The values are available to epoch processing and block processing. Now no extra loop over the validators tree is necessary to initiate exits. Those constants are copied on epochCtx.copy() so they can be safely mutated on different forks.

[codeclimate]

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[github-actions]

Performance Report

✔️ no performance regression detected

🚀🚀 Significant benchmark improvement detected

Benchmark suite	Current: 7c6528a	Previous: 485ec69	Ratio
Process block - 250000 vs - 7PWei - with 1 validator exit	5.2830 ms/op	118.28 ms/op	0.04
Process block - 250000 vs - 7PWei - with 16 validator exits	8.6390 ms/op	32.546 ms/op	0.27

Full benchmark results

Benchmark suite	Current: 7c6528a	Previous: 485ec69	Ratio
getCommitteeAssignments - req 1000 vs - 250000 vc	7.9663 ms/op	9.2282 ms/op	0.86
epoch altair - 250000 vs - 7PWei - processInactivityUpdates	2.3855 s/op	3.0492 s/op	0.78
epoch altair - 250000 vs - 7PWei - processRewardsAndPenalties	1.1170 s/op	1.2325 s/op	0.91
epoch altair - 250000 vs - 7PWei - processParticipationFlagUpdates	361.55 ms/op	572.53 ms/op	0.63
Process block - 250000 vs - 7PWei - with 0 validator exit	5.2932 ms/op	13.147 ms/op	0.40
Process block - 250000 vs - 7PWei - with 1 validator exit	5.2830 ms/op	118.28 ms/op	0.04
Process block - 250000 vs - 7PWei - with 16 validator exits	8.6390 ms/op	32.546 ms/op	0.27
epoch phase0 - 250000 vs - 7PWei - beforeProcessEpoch	1.6624 s/op	1.8831 s/op	0.88
epoch phase0 - 250000 vs - 7PWei - processRewardsAndPenalties	809.31 ms/op	954.61 ms/op	0.85
epoch phase0 - 250000 vs - 7PWei - processEffectiveBalanceUpdates	144.70 ms/op	188.89 ms/op	0.77
getAttestationDeltas - 250000 vs - 7PWei	133.96 ms/op	136.41 ms/op	0.98
processSlots - 250000 vs - 7PWei - 32 empty slots	3.4361 s/op	3.8874 s/op	0.88
shuffle list - 16384 els	17.527 ms/op	37.375 ms/op	0.47
shuffle list - 250000 els	235.62 ms/op	529.31 ms/op	0.45
getPubkeys - persistent - req 1000 vs - 250000 vc	16.544 us/op	22.626 us/op	0.73
BLS verify - blst-native	1.8636 ms/op	2.3864 ms/op	0.78
BLS verifyMultipleSignatures 3 - blst-native	3.8116 ms/op	4.8731 ms/op	0.78
BLS verifyMultipleSignatures 8 - blst-native	8.2007 ms/op	10.422 ms/op	0.79
BLS verifyMultipleSignatures 32 - blst-native	32.596 ms/op	37.910 ms/op	0.86
BLS aggregatePubkeys 32 - blst-native	41.060 us/op	54.187 us/op	0.76
BLS aggregatePubkeys 128 - blst-native	155.67 us/op	200.91 us/op	0.77
getAttestationsForBlock	90.173 ms/op	146.81 ms/op	0.61
validate gossip signedAggregateAndProof - struct	4.0376 ms/op	7.6318 ms/op	0.53
validate gossip signedAggregateAndProof - treeBacked	4.0776 ms/op	5.9595 ms/op	0.68
validate gossip attestation - struct	1.8604 ms/op	2.7016 ms/op	0.69
validate gossip attestation - treeBacked	1.9748 ms/op	2.8023 ms/op	0.70

by benchmarkbot/action